This invention relates to a signal processing method for control purposes, and more particularly to a method of detecting convergence of a signal on a steady-state value.
In many feedback control systems, it is useful for purposes of stability enhancement to determine Whether and when a system parameter being controlled has converged to a steady-state or quasi-steady-state value. When convergence is detected, for example, the feedback control gains may be modified, and/or the current control value may be stored for feed-forward control when similar operating conditions are re-encountered. However, the detection of steady-state convergence is not a trivial matter because the variability that occurs in a converged signal is often difficult to distinguish from the variability that occurs prior to convergence. And yet, it is important to identify convergence of the signal as soon as possible in order to achieve the stability enhancement, particularly in a system where the operating conditions can quickly change. Accordingly, what is needed is a method for quickly and reliably detecting steady-state convergence of a signal.
The present invention is directed to an improved method for detecting steady-state convergence of a signal by comparing a filtered version of the signal or its numerical derivative to a threshold over a given time interval, wherein a measure of the signal variability is used to tune the filter behavior. In a first embodiment of the invention, a derivative of the signal is filtered with a low-pass filter, and the cut-off frequency of the filter is adjusted in proportion to the measured variability of the signal. In a second embodiment of the invention, the signal is filtered With a high-pass filter, and the cut-off frequency of the filter is adjusted inversely with respect to the measured variability of the signal. In each case, the variability of the signal is measured by computing a differential of the signal and then smoothing the differential.